Windborne debris hazards

Windborne Debris

Wind borne debris flight is often broken down into three stages; motion initiation, flight, and impact. Our research team has looked at the initiation of motion of roof gravel from built up roof systems, the flight of compact debris in a turbulent wind field, and the aerodynamics of rood like debris such firebrands formed by twig or branch breakoff from burning vegetation.

Roof gravel blow-off conditions

The conditions under which loose laid aggregate on a built up roof structure is blow-off in severe storms was investigated by former research group member Arash Karimpour. Arash ran a series of wind tunnel tests to determine the critical wind speed above which blow-off will occur. Arash developed a new technique for measuring the critical wind speed and investigated the role of particle size and parapet height on the critical velocity for a two-dimensional rectangular cross-section building. The study showed that the addition of a small parapet to a building may actually reduce the wind speed required for blow-off to occur (Karimpour & Kaye 2012). The study then went on to investigate the rate of removal of aggregate the influence of parapets on holding aggregate with in the roof cavity
(Karimpour & Kaye 2013). We are starting a new project in January 2019 to undertake full scale testing of blow-off criteria using the FUI-NHERI Wall of Wind. The data will be compared to theoretical and computational models for blow-off. This approach will resolve some of the issues raised by Arash’s  work.

Compact debris flight

Compact debris is roughly defined as objects for which all their dimensions are approximately equal. Examples include gravel, and sand particles. It is typically assumed that lift forces in compact debris are negligible and flight is therefore, approximately two dimensional. Despite this relatively simple geometry the flight patterns can be quite complex. Arash Karimpour ran a series of Monte-Carlo simulations of compact debris flight to examine the influence of ambient turbulence on the flight distance of compact debris (Karimpour & Kaye 2012). Arash showed that ignoring ambient turbulence resulted in an underestimation of the aerodynamic forces on a piece of debris and that the RMS velocity should be used for flight modeling.

A follow on paper (Kaye 2015) looked at the flight trajectory of compact debris near its release point. It was shown that, even for the simple case of compact debris released into a steady uniform wind field the flight trajectory has an inflection. The flight trajectory was particularly complex for debris with a Tachikawa number close to one and that this was important because a Tachikawa number close to one is the motion initiation criteria for much loose laid compact debris.

Rod like debris aerodynamics

The research team has investigated the aerodynamics of rod-like debris flight. This study is motivated by the observation that many embers formed from burning trees are long thin cylinders. Understanding the aerodynamics of rod-like debris can improve the modeling of ember flight and enhance our understanding of firebrand spot fire generation. Ali Tohidi and a number of summer RAs (Leigh, Frank, Alexandra, and Meredith) have conducted series of experiments to look at the dynamics of falling cylinders. The results of these test will be used to assess the accuracy of current parameterizations of rod like
debris force coefficients. The results were published in JWEIA.